The present invention relates to a switching power unit which is preferably realized as a so-called AC/DC converter, and particularly to reducing a loss in an activating circuit thereof.
There is wide use of a switching power unit, used in a portable-compact electronic device, which switches a dc current obtained by rectifying and smoothing a commercial current at a high frequency such as several hundreds kHz, so as to convert the dc current into a desired voltage by means of a compact transformer with high efficiency.
FIG. 14 is an electronic circuit diagram of a typical switching power unit 1 of prior art. A pulse width modulation (PWM) system and of a flyback system are employed in the switching power unit 1. In the switching power unit 1, an ac voltage applied to a line between terminals p1 and p2 flows via (a) a filter circuit, constituted of a fuse 2, filter capacitors c1 and c2, a filter coil 1, which prevents EMI, and (b) a discharging resistor r1. Thereafter, the ac voltage is rectified and smoothed by a diode bridge 3 and a smoothing capacitor c3. A dc voltage obtained in this manner is applied to a dc circuit constituted of a primary winding n1 of a transformer n and a main switching element q which is an FET, and a main switching element q is switched at a high frequency by a control circuit 4 as described later, so that an induced voltage occurs in a secondary winding n2 of the transformer n when the main switching element q is OFF. The induced voltage is rectified and smoothed by a diode d1 and a smoothing capacitor c4 so as to be outputted from terminals p3 and p4 to a load which is not shown.
When a user pulls an AC plug out from a socket for example, there is a possibility that the user gets an electric shock while filter capacitors c1 and c2 are kept to be charged. Thus, there is provided the discharging resistor r1 so as to discharge the filter capacitors c1 and c2 quickly, thus preventing an accident.
An output voltage detecting circuit 5 monitors an output voltage from terminals p3 and p4, and sends the detected output voltage from a photodiode d2 of a photo-coupler pc to a phototransistor tr, so as to feedback the output voltage to the control circuit 4 on the primary side. The control circuit 4 controls a duty of the main switching element q in accordance with data of the output voltage that has been fedback, so that an output voltage of the switching power unit is stabilized.
While, there is provided a capacitor c5 as a power source of the control circuit 4. The dc current that has been rectified and smoothed by the diode bridge 3 and the smoothing capacitor c3 is applied to the capacitor c5 via the activating resistors r2 and r3. Thus, when a dc power source is connected to the terminals p1 and p2 so as to activate the switching power unit 1, a charging voltage of the capacitor c5 gradually increases. When the charging voltage becomes a voltage at which the control circuit 4 begins to operate, the control circuit 4 begins to operate so as to send a control signal to the main switching element q, so that the aforementioned switching operation is started.
Further, a voltage induced in a sub winding n3 of the transformer n is applied to the capacitor c5 via the diode d3. Also in this manner, the capacitor c5 is charged. Thus, after the activation, the control circuit 4 keeps on operating by using the current supplied via the diode d3 as a power source. Furthermore, there is provided a protecting zener diode zd on the capacitor c5 in parallel to each other so that the charging voltage of the capacitor c5 applied by the activating resistors r2 and r3 does not exceed a rated voltage in a low-power-consumption mode of the control circuit 4.
The low-power-consumption mode is a mode in which an operation of the control circuit 4 is stopped so that a low charging voltage of the capacitor c5 does not cause the control circuit 4 to malfunction when power is turned on. In the low-power-consumption mode, a low voltage malfunction preventing circuit that is internally provided in the control circuit 4 stops an operation of the control circuit 4 in a period since power is turned on until the charging voltage of the capacitor c5 becomes a predetermined voltage at which the control circuit 4 begins to operate. During the period in which the operation is stopped, the control circuit hardly consumes a current. Further, even though a power voltage becomes lower than an operation low limit voltage due to any causes during the steady-state operation of the switching power unit, the low voltage malfunction preventing circuit works, so that the switching power unit becomes in the low-power-consumption mode. Then, the voltage at which the operation is started is set to be higher than the operation low limit voltage, so that a stable operation is ensured.
A current detecting resistor r4 is connected to the main switching element q in series. A current flowing in the main switching element q is converted into a voltage by the current detecting resistor r4. The converted voltage is inputted as a voltage Va, via a noise-removing low pass filter constituted of the resister r5 and a capacitor c6, to a current detecting terminal a of the control circuit 4 which is used to detect an overcurrent. When a peak of the voltage Va exceeds a predetermined overcurrent detecting level, the control circuit 4 performs an overcurrent protecting operation for stopping the switching operation of the main switching element q. This prevents a damage in the main switching element q which is brought about by a reason for which an output current higher than a rated current is drawn from the terminals p3 and p4. At this time, an operation mode of the control circuit 4 shifts to the low-power-consumption mode, so that a power consumption in the control circuit is greatly reduced.
The low pass filter is inserted so as to prevent malfunction of the control circuit 4 which is brought about by a reason for which a sharp current flowing upon turning on the main switching element q causes the voltage Va to exceed a predetermined overcurrent detecting level.
Further, the capacitor c6 is charged with a dc voltage from the smoothing capacitor c3 via bias resistors r6 and r7, and the capacitor c6 is discharged by the resistor r5 and the current detecting resistor r4 when the main switching element q is turned off. Thus, the bias resistors r6 and r7 charge the capacitor c6 so as to have a voltage Va calculated by the following expression, so that the current detecting terminal a of the control circuit 4 is biased by the voltage Va.
Va={square root over (2)}Vacxc3x97(r5+r4)/(r6+r7+r5+r4) 
Then, a voltage increment of the current detecting circuit r4 which is brought about by a current of the main switching element q is added to the right side of the foregoing expression, and the voltage is applied, as an increment of the voltage Va, to the current detecting terminal a of the control circuit 4 as described above.
Thus, when the main switching element q remains ON for a time more than a predetermined time due to output short circuit etc. so as to keep on charging the capacitor c6, the voltage Va exceeds the overcurrent detecting level due to the charging performed by the bias resistors r6 and r7, so that a short circuit protecting operation, by which the main switching element q is OFF-driven, is performed.
Further, in a case where there are not provided the bias resistors r6 and r7, there occurs the following problem: if the overcurrent detecting level is set with respect to an input ac voltage of 100 V for example, the overcurrent protecting operation is not performed unless the overcurrent detecting level is more than the set overcurrent detecting level when the input ac voltage is changed to 200 V. Thus, the bias resistors r6 and r7 are provided and the bias voltage value is changed according to the input ac voltage, so that it is also possible to make the overcurrent detecting level substantially fixed.
A series circuit constituted of the diode d4 and the capacitor c7 is connected to the primary winding n1 of the transformer n in parallel to each other so as to absorb a counter-electromotive force which occurs in the primary winding n1 when the main switching element q is OFF. Further, the resistor r8 is connected to the capacitor c7 in parallel to each other so as to consume a charge that has been accumulate in the capacitor c7 temporarily.
In the switching power unit 1 arranged as described above, when the overcurrent protecting operation is realized in accordance with the current detecting resistor r4, there occurs the following problem: in a case where 200 V is inputted compared with a case where 100 V is inputted, a difference between ac voltages inputted to the terminals p1 and p2 causes a current flowing in the main switching element q to be reduced by substantially half. Therefore, at the overcurrent detecting level in the case of 100 V, the protecting function does not work in the case of 200 V. Thus, in order to compensate the influence exerted by the difference in the input ac voltage, the current detecting terminal a is biased by the bias resistors r6 and r7 as described above so as to change the bias amount according to the input ac voltage value.
However, not only the bias resistors r6 and r7 but also the activating resistors r2 and r3 are connected to an output terminal of a dc voltage from the diode bridge 3 and the smoothing capacitor c3 both of which have high voltages, so that there occurs the following problem: power conversion efficiency of the switching power unit is deteriorated due to a large power consumption brought about by the resistors r2, r3; r6, r7. Further, although some of other prior arts are arranged so that the activating resistors r2 and r3 are connected to an AC line, that is, a line connected to the terminals p1 and p2, they have substantially the same arrangement as the foregoing prior art in that the activating resistors r2 and r3 are connected to a high voltage portion.
The object of the present invention is to provide a switching power unit which can reduce a loss in an activating circuit.
In order to achieve the foregoing object, the switching power unit of the present invention includes: a main switching element for performing switching with respect to a dc voltage that has been obtained by rectifying an input ac voltage with a rectifying circuit; a control circuit for controlling the switching according to output voltage data fedback from a secondary side so as to stabilize an output voltage at a desired value; a plurality of series resistors provided on an input side of the rectifying circuit; and an activating resistor for supplying an activating current from a connection point between the series resistors to the control circuit.
In a case where a dc voltage rectified by the rectifying circuit is smoothed and used as a power source of the control circuit, supposing that a peak value of an input ac voltage, that is, a root means square value of the input ac voltage is Vac, a voltage applied to the control circuit is a de voltage of {square root over (2)} Vac. On the other hand, in case of using a voltage which is smoothed after being drawn from the connection point between the series resistors provided on the input side, the voltage applied to the control circuit takes a dividing value that has been obtained by dividing a root means square value of a rough input ac voltage by means of the series resistors. Here, supposing that dividing number is N, the voltage applied to the control circuit is |({square root over (2)} Vacxc3x97sinxcfx89t)/N|, and when N=2 for example, |Vacxc3x97sinxcfx89t)/{square root over (2)}|.
Thus, the loss brought about by the activating resistor is reduced, so that it is possible to improve a power conversion efficiency of the switching power unit.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.